A U.N. van is seen damaged by an improvised explosive device (IED) outside the U.N. compound in Garowe, the administrative capital of Somalia's semi-autonomous Puntland, April 20, 2015.Feisal Omar

LONDON (Thomson Reuters Foundation) - Respect for United Nations and Red Cross staff is "disappearing", the U.N. aid chief has said following the killing of four U.N. workers in Somalia, as other agencies raised concern about aid workers in Yemen and South Sudan.

Six people, including four United Nations Children's Fund (UNICEF) staff, were killed when their minivan was hit by a roadside bomb in Somalia's semi-autonomous Puntland region on Monday.

"Aid workers are increasingly targets, with serious consequences for our ability to reach people who urgently need help," said Valerie Amos, the U.N. emergency relief coordinator.

The attackers have "violated every principle of humanity and common decency," she said in a statement late on Tuesday. "When aid workers are attacked, they are unable to help people."

Six International Medical Corps staff in Yemen were injured by an air strike on Monday, and the United Nations World Food Programme (WFP) said on Wednesday it was "deeply concerned" about three missing workers in South Sudan.

Attacks on humanitarian workers have increased every year for more than a decade, reaching a record of 264 attacks affecting 474 aid workers in 2013, according to U.N. figures.

Al Qaeda-linked al Shabaab has staged a string of attacks in Somalia and neighbouring countries aimed at imposing its harsh brand of Islam and overthrowing the Somali government, which is backed by Western donors and African peacekeepers.

The UNICEF staff were killed by an improvised explosive device while travelling to their office from their guest house nearby and the blast seriously wounded four other UNICEF staff, the organisation said in a statement.

"Attacks on humanitarian workers can constitute a war crime and are in total violation of international humanitarian and human rights law," Amos said.

ON THE FRONT LINES

The six International Medical Corps staff injured by an air strike in Sanaa, Yemen, were working in their office, the charity said late on Tuesday, adding that a warehouse containing medicine and relief supples was also damaged.

These "heroic" Yemeni men and women "now find themselves on the front lines of this fight," said Jon Cunliffe, Yemen director of International Medical Corps.

As Saudi Arabia said it was ending a month-long campaign of air strikes against Yemen's Houthi rebels on Tuesday, the World Health Organization said 944 people were reported killed and 3,487 wounded in Yemen in the four weeks up to Friday.

The three WFP workers missing in South Sudan vanished on April 1 en route to a food distribution, after fighting broke out on the road along which they were travelling, the WFP said in a statement on Wednesday.

"The disappearance comes amid generally deteriorating security and increasing harassment of humanitarian workers throughout the country," the WFP said, and WFP chief Ertharin Cousin said she was "extremely worried".

Another WFP staff member, Mark Diag, has not been seen since he was abducted in October 2014. The United Nations says 10 humanitarian workers have been killed in Upper Nile State since the start of South Sudan’s conflict more than 16 months ago.

A court in Guinea has sentenced 11 people to life in prison for murdering a team educating local people about the risks of Ebola in a remote part of the West African country last year, a state prosecutor said on Wednesday.

Last month three Red Cross and Red Crescent workers were killed within hours of each other in Mali, Yemen and Syria in attacks condemned as "profoundly shocking and unacceptable" by Elhadj As Sy, secretary general of the International Federation of Red Cross and Red Crescent Societies.